Your resting heart rate will drop as you become fitter, but you don't wan't to be dropping your training heart rate as you improve. For this reason I'd use the maximum heart rate and work back from that. Depending what you want to achieve will depend on the % of HRM you will be aiming for.

I don't train by heart rate anymore, but when I did I realized fairly quickly that I actually needed to do a proper test, as none of the formulas seemed to work for me. At the time I had a coach who asked me do a Cooper test with a heart rate monitor and from that he calculated my zones.
However, after a few weeks I figured out that the four zones corresponded precisely to my "very easy", "cruising speed", "making an effort" and "race speed" classification and that I could dial in without the monitor.

In reply to MikeTS: You want to be aiming between 65-75% of your HRM as a ballpark figure.

Disclaimer: It also epends on other factors such as current weight and health conditions etc. but presuming that you are reasonably healthy and just carrying a few extra lbs, then that's a good target zone to aim for for both weigh loss and endurance as a start.